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host or DNS message delivery

Posted on 2002-06-12
Last Modified: 2013-11-15
hope this is enough points, bit broke being new

in exchange 5.5 i was having problems with some emails being bounced back (only  about 3 addresses), yet these email addresses were recieving fine from anywhere else.
Now BT said make sure the internet mail service in exchange, under connections and message delivery, was set to forward all msgs to host, not DNS (use domain name system)
Any ideas why this worked, im pleased it did but would like to know why? as its 99% worked ok before
thanks for any replies
Question by:Mistertwigg
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LVL 12

Expert Comment

by:Wouter Boevink
ID: 7073668
If you use DNS exchnage server is doing a dns lookup on the domain of the reciever.

for example a mail to joe@somecompany.com

Exchange is looking up the MX records of somecompany.com and then sending the mail to the address returned from the MX records.

If you forward the message to a host, the host is doing the same, except in your case they do a better job 'cause it works. The only thing I can think of is that some addresses don't get correctly resolved. Can be a configuration error in your own DNS (if you have one).

LVL 55

Expert Comment

ID: 7076645
The answer is in the question: ---> BT

Do you have a PTR record for your IP address? half my BT customers don't but if you relay through BT's smarthost (which has a PTR for itself but none of it's clients) then servers that check for the presence of a PTR record still accept mail from you. Thus the 1% of domains that won't accept mail from your address (which had no reverse DNS entry since BT supply you with a broken netblock) will accept it if you relay it through their relay as that has a properly registered netblock assigned to their mailserver.

The crazy thing is that as a home adsl user I get a PTR record assigned to my dynamic IP but BT's corporate/leased line businesses don't get the same record created even though it's a static address. Don't care what the record says for a hostanme but I'm getting pissed off that no in-addr.arpa record exists if you have the business bundle.

Author Comment

ID: 7077612
id love to say i knew about ptr records, but i dont so i looked up a definition.
"Pointer Record. Also called a reverse record. A PTR record associates an IP address with a canonical name. PTR records should point to a name that can be resolved back to the IP address. The name of the pointer record is not the IP address itself, but is the IP address’ four IP octets in reverse order followed by IN-ADDR.ARPA. for Example:"

I sort of understand what it does but have no idea how its implemented, where, or by who, is it by BT, do i request it.
Thanks for you reply.
Mr Twigg

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Author Comment

ID: 7077619
is it something implemented within my exchange server (5.5)
LVL 55

Expert Comment

ID: 7077662
It's the ISP that has to impliment the record so in this case BT, to see if it is set run nslookup in a command prompt, lets pretend your mailserver is maila.microsoft.com. Note that NT's nslookup is user friendly and lets you enter the IP address instead of reversed-octets.in-addr.arpa

> maila.microsoft.com

Name:    maila.microsoft.com


Name:    mail1.microsoft.com


Name:    mail2.microsoft.com

So you see there is a PTR record for each of their mailservers so they will not get problems sending mail.

If you do the same for your IP addresses but get "can't find Non-existent domain" then the PTR is not set up. If there is no record you might find that some FTP sites won't let you connect either or take a long time as well.

All you can do is complain but I've been waiting for a month for them to create PTRs for one of my customers, they sub out the creation of the forward zones to another company and expect them to be able to create the reverse records which the subcontractor probably can't do.

Author Comment

ID: 7077788
Well explained thanks.
So why not forward all msgs to host (as ive now just changed it) instead of a DNS lookup. Is there an advantage to using DNS instead of forwarding all to host then letting them reslove the addys.
LVL 55

Accepted Solution

andyalder earned 20 total points
ID: 7077841
That's what I had to do for mail same as you but I don't like sending through relays, it's an extra point of failure and who's to say that the ISP's smarthost doesn't make a copy of every mail you send and give it to your corporate rivals? It is a bit quicker though if you have low bandwidth as your server doesn't have to do the DNS lookup.

My customer's problem is that to update their hosted web service they FTP to a host that checks for the presence of a PTR record before allowing a connection so at the moment their webmaster has to use a dialup modem for updates. Kind of defeats the object of paying for a leased line :(

Author Comment

ID: 7080657
thanks for the information, much appreciated

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